454 ENERGY MAY BE PROPAGATED BY VIBRATIONS A tone rich in overtones is carried to the ear by a mixture of compressional waves of different frequencies and different intensities, traveling in the same direction. There Is a Vast Range of Frequencies for Sound Waves. Pitch can be expressed in all terms of frequency by comparison with sounds created by mechanical means that produce a known number of vibrations per second. If a card is held against the serrated edge of a revolving disk, it will produce vibrations whose frequency depends upon the rate at which the disk moves. Of course, pitch can be obtained from records produced by the phonautograph, the phonodeik, and similar instruments, but it can also be determined by means of tuning forks which vibrate with known frequencies. Two sound waves whose frequencies are 250 and 251 per second will be in phase ^ at one instant and produce the maximum sound, but half a second later they will be opposite in phase, and less sound will be heard because of interference. At the end of the second they will be in phase again, and the sound will again be heard. If the frequencies had been 250 and 252, there would have been two of these variations of loudness, otherwise known as "beats." In comparing the pitch of a vibration whose pitch is not known with a known pitch, the difference in the frequency between the tones is equal to the number of beats heard per second. This method of tuning instruments is The charm of music produced sufficiently accurate for most purposes. by bells is due in part to the beats that accompany their tones. Middle C on the piano has a frequency of 258.7 complete vibrations (cycles) per second on the International Scale. An octave consists of the range of pitch between any two frequencies in which one is twice the other. The piano has a range from 27.2 to 4138.4 complete vibrations per second. The range for the organ is 16 to 4138 complete vibrations per second. The organ pipe giving 16 complete vibrations per second is about 32 feet long. A few organs have pipes 64 feet long that give only 8 complete vibrations per second. The organ pipe that has a frequency of 4138 complete vibrations per second is only 1 Yi inches long. There are upper and lower limits to the pitch of notes which can be heard by the ear, but both limits vary with different persons. Few 1 Phase — Imagine two boys swinging in two identical swings placed side by side, the period in each case being the same. If the two boys reach the same relative positions at the same time, they are in phase. If they are opposite each other when they reach their greatest height, there is a phase difference of 180°. while, if one boy is at the height of his swing while the other boy is at the lowest point of his swing, there is a phase difference of 90°. Any other combination of swings which are out of phase could be worked out.
SOUND PRODUCED BY VIBRATIONS IN MATTER 455 people can distinguish frequencies fewer than 12 to 16 per second or more than 20,000 per second. Frequencies above 5000 per second produce sounds that are mere squeaks or unpleasant shrill notes. Younger people can usually distinguish higher frequencies than can older people. Soundless Sound Waves Have Many Possible Applications. The frequencies above the upper limit of hearing are called "supersonic" vibrations; frequencies up to and beyond five million per second have been produced and measured. These higher frequencies are carried through the air at about the usual speed for sound waves, but they are absorbed by carbon dioxide. They may be transmitted through water and could be used as a means of communication between ships. Some experiments with powerful supersonic sound waves indicate that they have possibilities in reducing the smoke nuisance by causing smoke particles to coagulate. Sounds can bring about chemical reactions if the sounds are intense enough. Thus proteins have been coagulated, ethyl acetate has been broken down to produce acetic acid, vegetable oils have been cracked, and starch has been changed to glucose with sound waves. This is a subject that is certain to attract a great deal of research in the future. It is possible to create burns on the fingers by holding a glass rod which is dipped into ultrasonically vibrating oil, although the temperature of the oil is at ordinary room temperature. The glass conducts the sound to the fingers because it is a better conductor than is oil. The application of ultrasonic vibrations to an arm or leg will heat the marrow of the bone, although the bone remains at normal body temperature. Bone conducts vibrations, whereas flesh and marrow absorb them and, as a consequence, become heated. Milk and photographic emulsions may be homogenized by ultrasonic vibrations. Submarines may communicate with each other over distances ten to fifteen miles, using ultrasonic beams which can be concentrated within one degree of the desired direction. The Doppler Effect Is the Apparent Change in Frequency Produced by the Relative Motion of a Sound Source and an Observer. If the horn of a passing automobile is blown continuously as it approaches and passes an automobile moving in the opposite direction, the pitch will decrease as the automobile passes and recedes relative to the observer, and the change in pitch will depend upon the relative of